Electric-motor drives are known in various embodiments for focusing and adjusting the focal length of photographic objectives. In this regard, compact DC motors having a small construction, ultrasound motors, and stepper motors having gear mechanism units are primarily used.
Likewise, ultrasound motors that are configured as ring motors and disposed on the circumference of a holder of an objective are known as drives. Ring motors generally drive the focusing elements and focal length adjustment elements provided for axial adjustment in the axial direction, using a gear mechanism translation. They require a complicated electronic controller with high electrical voltages. Mechanical friction between stator and rotor causes contamination in the interior of the objective tube and impairs the image performance due to deposits on the optical elements.
An apparatus for an axial position change of an optical imaging system is known from document DE 197 18 189 A1. The optical imaging system is disposed within a carrying ring that is guided in a housing so as to be longitudinally displaceable. Magnet elements are disposed on the outer circumference surface of the carrying ring, which elements follow a magnetic field controllably disposed on the outer circumference of the housing, and thereby axially displace the carrying ring together with the imaging system.
From EP 1 884 813 A1, an electromagnetic drive for axial adjustment of an optical imaging system held in a holder is known. The electric motor drive consists of a coil arrangement wound parallel to the optical axis of the imaging system, and a permanent magnet that engages around the optical axis as an arc segment. The coil arrangement and the permanent magnet have a common iron back network. When current is applied to the coil winding, it moves over the arc segment of the permanent magnet and in doing so adjusts the holder of the optical imaging system by way of a pin/slot coupling guided in a curved support.